This invention relates generally to fluid flow control systems incorporating pinch valves through which fluid conduits are threaded and the valves operated alternatively to compress the conduit preventing flow and to release the conduit permitting flow.
More particularly, the invention provides a molded elastomeric flexible conduit having outwardly extending purchase means unitary therewith and adapted to be cooperatively coupled to suitable means carried by the valve to enable spreading of the tube when the valve was operated to permit flow subsequent to retention of the conduit in pinched, compressed condition. When the conduit is compressed for a prolonged duration, its elastomeric memory normally to return to the open condition is overcome. The pinched conduit remains closed, or at least partially closed, notwithstanding the relief of the compressive force. The conduit also can become stuck together so as to resist opening when same is desired.
The art has provided many different types of apparatus to perform automated analysis, with many resolutions of problems inherent in varying degree with the classical analytical methods. Many types involve the transfer of liquid in such a manner as to provide quantitatively accurate dilutions and physical transfer of precise volumes of liquid from one location to others.
Difficulties are encountered in switching from one flow path to another. One transfer system involves the use of check valves which are normally one way directional flow valves. Such valves are subject to sealing or seating problems which may be caused by sedimentary buildup formed upon the sealing mechanism. Thus error is introduced due to the extra fluid flow past the sealing location. Other problems are encountered, such as backlash occurring due to time delay between pressure change to close the valve and the time the valve actually closes. Here, more liquid than desired may be permitted past the valve before it has an opportunity or the time within which fully to close off flow. Swelling is another general problem encountered with many of the known check valve structures.
In U.S. Pat. Nos. 3,882,889 and 3,932,065, a solution was offered to the above mentioned problems. There was provided a pneumatically operated pinch valve of the make-before-break type operable upon flexible conduit paths defined by flexible conduits. A valve shell or housing was provided with piston means slidable therein laterally. The conduits were passed through a suitable window formed in the shell and introduced into the path of the pistons. A stationary post also was provided cooperating with at least one of the pistons to stop flow in a conduit passed therebetween.
Notwithstanding the substantial benefit provided by the patented pinch valve and fluid transfer system employing such structures, problems have been encountered which were not fully addressed heretofore.
As mentioned earlier, the flexible conduits generally are formed of elastomeric plastic tubing. The elastomeric tubing possesses a memory to maintain its tubular configuration allowing passage therethrough. The memory normally causes the tubing to return to its open condition after it has been compressed, for example, as by pinching, for a prolonged time period. If compressed, the tubing loses its memory and fails quickly to return to its open condition after prolonged compression. Cold flow may be at least partially responsible for the tube's loss of memory when compressed for a prolonged time period. The loss of memory often is neither predictable nor desirable. The tube can become stuck in closed condition due to the fluid being transported therein.
Pinch valve structures often included a manual override in the form of cam-lock arrangement whereby the valve could be placed in inoperative condition, the conduits remaining open. Although the functional advantages of this proposed solution were satisfactory using a cam-lock arrangement but added increased fabrication and assembly costs. Further, there are situations where one desires to maintain the flexible conduits, one or more thereof, in pinched closed condition for a prolonged duration. Yet when operation is desired, instantaneous response is desired.